In electrical circuits, there is often a need for allowing electric current to flow in one direction but not the other. Diodes provide this functionality and are ubiquitous in modern electronics. A diode commonly refers to a two-terminal electronic component that conducts primarily in one direction (e.g., has an asymmetric conductance between the two terminals, often referred to as electrodes). Over the years, diodes have taken a variety of forms, from thermionic diodes (based on vacuum tubes) to semiconductor-based diodes using point contacts or n-p junctions.
However, conventional diodes, even in the high conductance direction, have non-zero resistances. Thus, applications of conventional diodes have been limited.